Methods of forming high aspect ratio plated through holes and high precision stub removal in a printed circuit board

ABSTRACT

The present invention relates to printed circuit boards (PCBs), and more particularly, to methods of forming high aspect ratio through holes and high precision stub removal in a printed circuit board (PCB). The high precision stub removal processes may be utilized in removing long stubs and short stubs. In the methods, multiple holes of varying diameter and depth are drilled from an upper and/or lower surface of the printed circuit board utilizing drills of different diameters.

CLAIM OF PRIORITY UNDER 35 U.S.C. § 119

The present Application for Patent is a divisional of U.S. patentapplication Ser. No. 14/603,191 filed Jan. 22, 2015, issuing on Jan. 22,2019 as U.S. patent Ser. No. 10/188,001, which claims priority to U.S.Provisional Application No. 61/930,456 entitled “METHODS OF FORMING HIGHASPECT RATIO PLATED THROUGH HOLES AND HIGH PRECISION STUB REMOVAL IN APRINTED CIRCUIT BOARD”, filed Jan. 22, 2014, assigned to the assigneehereof and hereby expressly incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to printed circuit boards (PCBs), and moreparticularly, to methods of forming high aspect ratio through holes andhigh precision stub removal in a printed circuit board (PCB).

BACKGROUND

Consumers are increasingly demanding both faster and smaller electronicproducts. The use of PCBs has grown enormously as new electronicapplications are marketed. A PCB is formed by laminating a plurality ofconducting layers with one or more non-conducting layers. As the size ofa PCB shrinks, the relative complexity of its electricalinterconnections grows.

A via structure is traditionally used to allow signals to travel betweenlayers of a PCB. The plated via structure is a plated hole within thePCB that acts as a medium for the transmission of an electrical signal.For example, an electrical signal may travel through a trace on onelayer of the PCB, through the plated via structure's conductivematerial, and then into a second trace on a different layer of the PCB.

Unfortunately, due to limitations within the prior art, the plated viastructure may be longer than necessary to perform the function ofelectrical connectivity. For example, the plated via structure mayextend completely through the PCB but only connect two traces on twoproximate adjacent layers. As a result, one or more stubs may be formed.A stub is excessive conductive material within the plated via structurewhich is not necessary to transport the electrical signal.

When a high speed signal is transmitted through the plated viastructure, a “stub effect” may distort the signal. The stub effect is aresult of the useless excess conductive material present within theplated via structure. The stub effect occurs when a portion of thesignal is diverted away from the trace connections and into one or morestubs of the plated via structure. The portion of the signal may bereflected from the end of the stub back toward the trace connectionsafter some delay. This delayed reflection may interfere with signalintegrity and increase, for example, the bit error rate of the signal.The degenerating effect of the stub effect may increase with the lengthof the stub.

FIGS. 1A-1E illustrate the typical stages of forming back-drilled holein a printed circuit board (PCB). As shown, the PCB 100 includes stackedmaterial insulator layers (typically laminates and prepregs) 104 a, 104b . . . 104 f separating five internal layers 105 a, 105 b . . . 105 eand two external layers 108 a and 108 b. The internal layers 105 a, 105b and 105 e are signal layers and the internal layers 105 c and 105 dare plane layers. The PCB 100 has an upper surface 110 a and an opposinglower surface 110 b.

To form the plated through hole 101 as illustrated in FIG. 1E, a firsthole 102 having a first diameter d1 is drilled through the PCB 100 (seeFIG. 1A). Next, a second hole 103 having a second diameter d2 is drilledconcentrically around and through a predetermined depth of the firsthole 102 (see FIG. 1B). Then, the walls of the remaining portion of thefirst hole 102 and the walls of the second hole 103 are plated with athin layer of a conductive material 106 (e.g., copper). (See FIG. 1C).

Next, a drill 112 having a diameter d3 is used to back-drill and form athird hole 104 concentrically around and through the remaining portionof the plated first hole 402 starting from the lower surface 110 b ofthe printed circuit board 100 (see FIG. 1D). Then the drill 112 isremoved from the printed circuit board 100 which now has the via 101formed therein that includes a first through hole 107 (e.g., platedsecond hole 103) and a back-drilled hole 109 (e.g., third hole 104) (seeFIG. 1E).

However, when back-drilling the drill bit has no pressure and it pushesup against the traces on the top and as a result fluctuating is createdas well as peel off of the conductive plating occurs. (See FIGS. 1D-1E)Consequently, there is a need for improved methods for removing stubswhen forming plated through holes in a PCB.

SUMMARY

The following presents a simplified summary of one or moreimplementations in order to provide a basic understanding of someimplementations. This summary is not an extensive overview of allcontemplated implementations, and is intended to neither identify key orcritical elements of all implementations nor delineate the scope of anyor all implementations. Its sole purpose is to present some concepts ofone or more implementations in a simplified form as a prelude to themore detailed description that is presented later.

According to one aspect, a method for forming a plated through hole in aprinted circuit board is provided. The method includes drilling a firsthole having a first diameter through an upper surface of the printedcircuit board; drilling a second hole having a second diameter throughthe first hole to a lower surface of the printed circuit board, thelower surface opposite the upper surface; plating the first hole and thesecond hole with a conductive material; and drilling a third hole havinga third diameter through the first hole and the second hole. The printedcircuit board includes a plurality of plated through holes formedtherein.

In one example, the first hole is drilled a predetermined depth and thepredetermined depth is shorter than half the vertical distance of theprinted circuit board. The second diameter is smaller than the firstdiameter, and the third diameter is larger than the second diameter andsmaller than the first diameter. The third hole is drilled from theupper surface to the lower surface of the printed circuit board.

In another example, the second diameter is larger than the firstdiameter, and the third diameter is larger than the second diameter andsmaller than the first diameter.

In yet another example, the second diameter is smaller than the firstdiameter; a top of the third hole to a bottom of the second hole remainsundrilled; and the third diameter is larger than the second diameter.

In yet another example, the method further includes adding copper to anyremaining conductive surface by electrolytic plating.

In yet another example, the second diameter is smaller than the firstdiameter; a top of the third hole to a bottom of the first hole remainsundrilled; and the third diameter is larger than the second diameter.The method further includes adding copper to any remaining conductivesurface by electrolytic plating.

In yet another example, the method further includes embedding a firstconductive material in the printed circuit board

According to another aspect, a method for forming a plated through holein a printed circuit board is provided. The method includes drilling afirst hole having a first diameter from a lower surface of the printedcircuit board; drilling a second hole having a second diameter throughan upper surface of the printed circuit board to the first hole of theprinted circuit board, the upper surface opposite the lower surface;plating the first hole and the second hole with a conductive material;and drilling a third hole having a third diameter through the first holeand the second hole. The printed circuit board has a plurality of platedthrough holes formed therein.

According to one example, the second hole is drilled a predetermineddepth; and the predetermined depth is shorter than half the verticaldistance of the PCB. The second diameter is larger than the firstdiameter; and the third diameter is larger than the second diameter andsmaller than the first diameter. The third hole is drilled from theupper surface to the lower surface of the printed circuit board.

According to another example, the second diameter is larger than thefirst diameter; and the third diameter is smaller than the seconddiameter and larger than the first diameter.

According to yet another example, the second diameter is larger than thefirst diameter; a top of the third hole to a bottom of the second holeremains undrilled; and the third diameter is larger than the firstdiameter.

According to yet another example, the method further includes addingcopper to any remaining conductive surface by electrolytic plating.

According to yet another example, the first hole is drilled apredetermined depth; and the predetermined depth is shorter than halfthe vertical distance of the printed circuit board. The method furtherincludes plating the third hole with a conductive material forming aplated through hole; wherein the second hole is drilled to a top of thefirst hole; and the third diameter is larger than the first diameter andthe second diameter.

According to yet another example, a method for forming a plated throughhole in a printed circuit board, including a first surface and anopposing second surface, is provided. The method includes embedding afirst conductive material in the printed circuit board; drilling a firsthole having a first diameter from a first surface of the printed circuitboard to a first predetermined depth; drilling a second hole having asecond diameter through the first hole to a second predetermined depth,plating the first hole and the second hole with a second conductivematerial; and drilling a third hole having a third diameter through thefirst hole and the second hole. The printed circuit board has aplurality of plated through holes formed therein; and wherein theconductive material is copper.

According to one example, the first surface is an upper surface and thesecond surface is a lower surface; the first predetermined depth is fromthe upper surface to the embedded first conductive material; the secondpredetermined depth is from the first surface to the second surface;wherein second diameter is smaller than the first diameter; and thethird diameter is larger than the second diameter and smaller than thefirst diameter.

According to another example, the first surface is a lower surface andthe second surface is an upper surface; the first predetermined depth isfrom the lower surface through the embedded first conductive material;the second predetermined depth is from the first surface to the firsthole of the printed circuit board; and the second diameter is smallerthan the first diameter; and the third diameter is larger than thesecond diameter and smaller than the first diameter.

According to yet another example, the first surface is a lower surfaceand the second surface is an upper surface; the first predetermineddepth is from the lower surface through the embedded first conductivematerial; the second predetermined depth is from the first surface tothe first hole of the printed circuit board; and the second diameter issmaller than the first diameter; and the third diameter is larger thanthe first diameter and smaller than the second diameter.

According to yet another example, the first surface is a lower surfaceand the second surface is an upper surface; the first predetermineddepth is from the lower surface through the embedded first conductivematerial; the second predetermined depth is from the upper surface tothe printed circuit board through the embedded first conductivematerial; the second diameter is smaller than the first diameter; andthe third diameter is smaller than the second diameter and larger thanthe second diameter.

According to yet another example, the method includes adding copper toany remaining conductive surface by electrolytic plating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate the typical stages of forming back-drilled holein a PCB.

FIGS. 2A1, 2A2 and 2B-2D illustrate the different fabrication stages forforming a plated through hole (or via) and removal of a long stub in aprinted circuit board (PCB), according to one aspect.

FIGS. 3A-3G are illustrations of the different fabrication stages forforming a plated through hole (or via) and removal of a long stub in aprinted circuit board (PCB), according to one aspect.

FIGS. 4A-4D are illustrations of the different fabrication stages forforming a plated through hole (or via) and removal of a long stub in aprinted circuit board, according to one aspect.

FIGS. 5A-5D illustrate the different fabrication stages for forming aplated through hole (or via) and removal of a long stub in a printedcircuit board, according to one aspect.

FIG. 6 illustrates a method for forming a plated through hole andremoving a long stub in a laminated PCB, according to one aspect.

FIG. 7 illustrates a method for forming a plated through hole andremoving a long stub in a laminated PCB, according to one aspect.

FIG. 8 illustrates a method for forming a plated through hole andremoving a long stub in a laminated PCB, according to one aspect.

FIG. 9 illustrates a method for forming a plated through hole andremoving a long stub in a laminated PCB, according to one aspect.

FIGS. 10A-10G are illustrations of the different fabrication stages forforming a plated through hole (or via) and removal of a short stub in aprinted circuit board (PCB), according to one aspect.

FIGS. 11A1, 11A2 and 11B-11G are illustrations of the differentfabrication stages for forming a plated through hole (or via) andremoval of a short stub in a printed circuit board (PCB), according toone aspect.

FIG. 12 illustrates a method for forming a plated through hole in alaminated PCB, according to one aspect.

FIG. 13 illustrates a method for forming a plated through hole in alaminated PCB, according to one aspect.

FIG. 14 illustrates a method for forming a plated through hole in alaminated PCB, according to one aspect.

FIG. 15 illustrates a method for forming a plated through hole in alaminated PCB, according to one aspect.

FIGS. 16A-16H illustrate the different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB and a conductive material filled core having a connecting innerlayer trace (target layer) for forming a via without a stub, accordingto one aspect.

FIGS. 17A-17H illustrate the different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB and a conductive material filled core having a connecting innerlayer trace (target layer) and adjacent conductive material filledprepreg for forming a via without a stub, according to one aspect.

FIGS. 18A-18H illustrate of the different fabrication stages for forminga plated through hole (or via) using different diameter drills within aPCB having a conductive material filled prepreg between conductivematerial filled cores; and having a connecting inner layer trace (targetlayer) for forming a via without a stub, according to one aspect.

FIGS. 19A-19H illustrate different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB having a conductive material filled prepreg and solid core as wellas having a connecting inner layer trace (target layer) for forming avia without a stub, according to one aspect.

FIGS. 20A-20H illustrate different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB having a conductive material filled prepreg between a conductivematerial filled core and a solid core; and a connecting inner layertrace (target layer) for forming a via without a stub, according to oneaspect.

FIGS. 21A-21H illustrate different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB and multiple conductive material filled cores having a connectinginner layer trace (target layer) and adjacent conductive material filledprepregs for forming a via without a stub, according to one aspect.

FIGS. 22A-22G are illustrations of the different fabrication stages forforming a plated through hole (or via) having a high aspect ratio in aprinted circuit board (PCB).

FIG. 23 illustrates a method for forming a plated through hole, having ahigh aspect ratio, in a PCB, according to one aspect.

FIGS. 24A-24D illustrate a method of forming a plated through hole,having a high aspect ratio, in a printed circuit board, according to oneaspect.

DETAILED DESCRIPTION OF THE INVENTION

While the present disclosure provides methods for forming plated throughholes in multi-layer printed circuit boards, the present disclosure isnot restricted to us in PCBs. A multilayer PCB can be a packagesubstrate, a motherboard, a line card, a puddle card, a backplane, amidplane, a flex or rigid flex circuit. A via structure can be a platedthrough hole (PTH) used for transmitting electrical signals from oneconducting layer to another. A plated via structure can also be acomponent mounting hole for electrically connecting an electricalcomponent to other electrical components on the PCB.

Removal of Long Stub

FIGS. 2A1, 2A2 and 2B-2D illustrate the different fabrication stages forforming a plated through hole (or via) and removal of a long stub in aprinted circuit board (PCB), according to one aspect. As describedbelow, the final drill direction prevents the peeling up of the cuttingedge of the plated corner.

The location where a signal is transmitted through the PTH, or where afirst layer 202 is connected to a second layer 204 in the PCB 200, maybe represented as a target layer (T). According to one aspect, the firstlayer 202 may include a hole 206 drilled using a long depth drill whilethe second layer 204 may include an upper hole 203 drilled using a shortdepth drill. The long depth drill has a length that is less than orequal to the distance from the bottom of the first layer 202 to thetarget layer T prior to plating.

As the relative drill accuracy to PCB layers can depend on the drilldepth length, drilling a hole having shorter depth provides betteraccuracy then drilling a hole having a longer depth. As shown in FIG.2A-1, the upper hole 203 is formed using the short depth drill and has adiameter of d1 and a depth length L1, such that the bottom of the upperhole 203 a is below the target layer T. In other words, the short depthdrill passes through the target layer T when drilling from the secondlayer 204 downward.

Next, as shown in FIG. 2B, the long depth drill may be used to drill alower hole 206 up to and through the upper hole 203 forming a completehole through the PCB, or the long depth drill is performed downward fromhole 203 to the other PCB surface. The lower hole 206 has a diameter d2and a length L2 where d2 is less than d1 and L2 is greater than L1.

Alternatively, as shown in FIG. 2A-2, the lower hole 206 may be drilledfirst, using the long depth drill, up to the Target Layer T. Next, thesmall depth drill may be used to drill the upper hole 203 such that itconnects with the lower hole 206 at or below the target layer T. (SeeFIG. 2B)

After the dual diameter hole (i.e. the upper hole 203 having a diameterd1 and lower hole 206 having a diameter d2) has been formed, the dualdiameter hole may then be plated by a conductive material 207. (See FIG.2C) Next, a middle drill, having a diameter d3, where d3 is greater thand2 but less than d1, may be used to remove plated conductive materialfrom the surface of the lower hole 206 by drilling down through theupper hole 203 and drilling the conductive material on the lower hole206.

Referring to FIGS. 3A-3G, different fabrication stages for forming aplated through hole (or via) 300 (FIG. 3G) in a printed circuit board(PCB) 301 are illustrated. The PCB 301 has an upper surface 301 a and anopposing lower surface 301 b. The through hole 300 has an upperconductive portion 302 and a lower non-conductive portion 304.

A three (3) drill step process may be used to form the plated throughhole 300. In the first step, a first hole 306 is drilled, from the uppersurface 301 a of the PCB, using a first drill 308 having a firstdiameter d1. The first hole 306 is drilled to a predetermined depth,such as just past a target layer T in the PCB 301. (See FIGS. 3A and 3B)

Next, a second hole 310 may be drilled, using a second drill 312 havinga second diameter d2, where d2 is smaller than d1. (See FIGS. 3C, and3D) The interior surface of the holes 306 and 310 may then be plated 311with a conductive material. (FIGS. 3E and 3F)

Referring to FIG. 3F, after the first and second holes 306 and 310 areplated 311, a third drill 314, having a third diameter d3, may pass(drill) through the first and second holes 306 and 310. The thirddiameter d3 is larger than the second diameter d2 but smaller than thefirst diameter d1. As the third drill 314 has a larger diameter then thesecond hole 310 but a smaller diameter than the first hole 306, theconductive material 311 in the first hole 306 may be left intact but theconductive material on the second hole 310 is removed, i.e. the stubarea is removed. As a result, the through hole 300, as shown in FIG. 3G,has an upper conductive portion 302 and a lower non-conductive portion304.

Alternatively, the sequence of the first and second drill processesdescribed above can be changed. In the first step, a first hole 310 maybe drilled, from the lower surface 301 b of the PCB, using a first drill312 having a first diameter d1. The first hole 310 may be drilled to apredetermined depth, such as just before the target layer in PCB 301 ordrilled through the entire PCB 301, such as shown in FIG. 2A-2.

Next, a second hole 306 may be drilled, from the upper surface 301 a ofthe PCB, using a second drill 308 having a second diameter d2, where d2is larger than d1. (See FIG. 2B) The interior surface of the holes 306and 310 may then be plated 311 with a conductive material. (FIGS. 3E and3F)

Referring to FIG. 3F, after the first and second holes 306 and 310 areplated 311, a third drill 314, having a third diameter d3, may pass (ordrill) through the holes first and second 306 and 310. The thirddiameter may be larger than the second diameter d2 but smaller than thefirst diameter d1. As the third drill 314 has a larger diameter then thesecond hole 310 but a smaller diameter than the first hole 306, theconductive material 311 in the first hole 306 may be left intact whilethe conductive material on the second hole 310 is removed, i.e. the stubarea is removed. As a result, the through hole 300, as shown in FIG. 3G,may include an upper conductive portion 302 and a lower non-conductiveportion 304.

FIGS. 4A-4D illustrate the different fabrication stages for forming aplated through hole (or via) and removal of a long stub in a printedcircuit board, according to one aspect. The location where a signal goesthrough the PTH, or where a first layer 402 is connected to a secondlayer 404 in the PCB 400, may be represented as a target layer (T).According to one aspect, the first layer 402 may include a first hole406, having a diameter d1, that is drilled through the first and secondlayers 402, 404 and a second hole 408, having a diameter d2, where d2 isgreater than d1, may be drilled from the top surface of the board 400 toslightly below the target conductive layer T. A seeding conductivematerial 410, such as electroless copper plating, is applied to thefirst hole 406 and the second hole 408. A third hole 412, having adiameter d3, which is greater than d1 but less than d2, is back drilledto slightly below the target layer T. As d3 is greater than d1, theseeding conductive material 410 in the second hole 408 is removed upondrilling the third hole 412. Next, electrolytic plating 414 is thenapplied to the second hole 408.

FIGS. 5A-5D illustrate the different fabrication stages for forming aplated through hole (or via) and removal of a long stub in a printedcircuit board, according to one aspect. The location where a signal goesthrough the PTH, or where a first layer 502 is connected to a secondlayer 504 in the PCB 500, is represented as a target conductive layer(T). According to one aspect, the first layer 502 may include a firsthole 506, having a diameter d1, drilled through the first and secondlayers 502, 504. A seeding conductive material 508, such as electrolesscopper plating, is then applied to the first hole 506. Next, a secondhole 510, having a diameter d2, where d2 is greater than d1, is backdrilled to slightly below the target conductive layer T. After backdrilling the second hole 510, electrolytic plating 512 is then appliedto the first hole 506.

FIG. 6 illustrates a method for forming a plated through hole andremoving a long stub in a laminated PCB, according to one aspect. In themethod, a first hole, having a first diameter, may be drilled, from anupper surface of the PCB, a predetermined depth in a printed circuitboard using a first drill having a first diameter 602. For example, thepredetermined depth may be half way through the PCB. Next, a secondhole, having a second diameter, may be drilled through the remainingportion of the PCB to the lower surface of the PCB 604 creating orforming a through hole. The second diameter may be smaller than thefirst diameter. Next, the first hole and the second hole may be platedwith a conductive material 606. Next, a third hole having a thirddiameter may be drilled through the first hole and the second hole fromthe upper surface to the lower surface of the PCB, the third diameter islarger than the second diameter and smaller than the first diameter 608.As the third diameter is larger than the second diameter and smallerthan the first diameter, when the third hole is drilled the platedmaterial may be removed from the inner surface of the second hole. Thatis, the stub may be removed. As a result, a through hole having an upperconductive portion and a lower non-conductive portion is formed.

In another aspect, FIG. 7 illustrates a method for forming a platedthrough hole and removing a long stub in a laminated PCB, according toone aspect. In the method, a first hole, having a first diameter, may bedrilled, from a lower surface of the PCB, a predetermined depth in aprinted circuit board using a first drill having a first diameter 702.For example, the predetermined depth may be half way through the PCB.Next, a second hole having a second diameter may be drilled through anupper surface of the PCB to the first hole of the PCB 704. The firstdiameter is smaller than the second diameter. Next, the first hole andthe second hole may be plated with a conductive material 706. Next, athird hole having a third diameter may be drilled through the first holeand the second hole from the upper surface to the lower surface of thePCB, the third diameter is smaller than the second diameter and largerthan the first diameter 708. As the third diameter is smaller than thesecond diameter and larger than the first diameter, when the third holeis drilled the plated material may be removed from the inner surface ofthe first hole. That is, the stub is removed. As a result, a throughhole having an upper conductive portion and a lower non-conductiveportion is formed.

In yet another aspect, FIG. 8 illustrates a method for forming a platedthrough hole and removing a long stub in a laminated PCB, according toone aspect. A first hole, having a first diameter, may be drilled, froman upper surface of the PCB, a predetermined depth in a printed circuitboard using a first drill having a first diameter 802. For example, thepredetermined depth may be half way through the PCB. Next, a second holehaving a second diameter may be drilled through the first hole to thelower surface of the PCB 804. The second diameter is smaller than thefirst diameter. Next, the first hole and the second hole may be platedwith a thin conductive material such as electroless plating catalyst orelectroless copper 806. Next, a third hole having a third diameter maybe drilled through the first hole and the second hole of the PCB, thethird diameter is larger than the second diameter and smaller than thefirst diameter 808. A conductor, such as copper, may then be added tothe remaining conductive area by electrolytic plating 810. As the thirddiameter is larger than the second diameter and smaller than the firstdiameter, the third hole may prevent plating up of the conductivematerial so as to remove the thin base conductor, for electrolyticplating, from the inner surface of the first hole. That is, the stub maybe prevented. As a result, a through hole having an upper conductiveportion and a lower non-conductive portion may be formed.

In yet another aspect, FIG. 9 illustrates a method for forming a platedthrough hole and removing a long stub in a laminated PCB, according toone aspect. In the method, a first hole, having a first diameter, may bedrilled, from a lower surface of the PCB, a predetermined depth in aprinted circuit board using a first drill having a first diameter 902.For example, the predetermined depth may be half way through the PCB.Next, a second hole having a second diameter may be drilled through anupper surface of the PCB to the first hole of the PCB 904. The seconddiameter is larger than the first diameter. Next, the first hole and thesecond hole may be plated with a thin conductive material such aselectroless plating catalyst or electroless copper 906. Next, a thirdhole having a third diameter may be drilled through the first hole andthe second hole of the PCB, the third diameter may be larger than thefirst diameter and smaller than the second diameter 908. A conductor,such as copper, may then be added to the remaining conductive area byelectrolytic plating 910. As the third diameter is larger than the firstdiameter and smaller than the second diameter, the third hole mayprevent plating up of the conductive material so as to remove the thinbase conductor, for electrolytic plating, from the inner surface of thefirst hole. That is, the stub generation may be prevented. As a result,a through hole having an upper conductive portion and a lowernon-conductive portion may be formed.

Removal of Short Stub

Referring to FIGS. 10A-10G, the different fabrication stages for forminga plated through hole (or via) 1000 in a laminated printed circuit board(PCB) 1001 are illustrated. The PCB 1001 may have an upper surface 1001a and a lower surface 1001 b. The through hole 1000 may have an upperconductive portion 1002 and a lower non-conductive portion 1004.

As described below, a three (3) drill step process may be used to formthe plated through hole. In the first step, a first hole 1006 may bedrilled, i.e. from the lower surface 1001 b of the PCB, to apredetermined depth using a first drill 1008 having a first diameter d1.The first hole 1006 may be drilled just past the target layer in the PCB1001. (See FIGS. 10A and 10B)

Next, a second hole 1010 may be drilled, from the upper surface 1001 ato the top of the first hole 1006, using a second drill 1012 having asecond diameter d2, where d2 is larger than d1. (See FIGS. 10C and 10D)The interior surface of the holes 1006 and 1010 may be then plated witha conductive material 1013. (FIGS. 10E and 10F)

Referring to FIG. 10F, after the first and second holes 1006 and 1000are plated 1011, a third drill 1014, having a third diameter d3, maypass (drill) through the first and second holes 1006 and 1010. The thirddiameter is larger than the first diameter d1. As the third drill 1014has a larger diameter then the first hole 1006 but a smaller diameterthan the second hole 1010, the conductive material in the second hole1010 may be left intact but the majority of the conductive material onthe first hole 1006 is removed, i.e. the stub area is removed. As shownin FIG. 10G, the third drill 1014 may not be drilled all the way throughthe plated first hole 1006 leaving a small portion or small stub 1016 ofthe conductive material on the inner surface of the first hole 1006. Asmall stub 1016 may remain because when drilling upwardly from the lowersurface 1001 b of the PCB, the drill bit has no pressure and createsfluctuating as well as copper peel off. Also, the short depth drill hashigher depth accuracy than the long depth drill, so controlling the stublength by using a shorter depth drill may provide shorter stub lengththan when using a longer depth drill. For instance, this process mayprovide a shorter stub than when a third drill is drilled from top hole1010 to bottom hole 1006 as in like FIG. 2 and FIG. 3 which iscontrolled stub length by the second drill. As a result, the throughhole 1000, as shown in FIG. 10G, may have an upper conductive portion1002, a lower non-conductive portion 1004 and a small stub 1016. (FIG.10G)

Alternatively, in the first step, a first hole 1010 may be drilled, i.e.from the upper surface 1001 a of the PCB, to a predetermined depth usingthe first drill 1012 having a first diameter d1. The first hole 1010 maybe drilled to just before the target layer in the PCB 1001. Next, asecond hole 1006 may be drilled, from the lower surface 1001 b to thebottom of the first hole 1010, using a second drill 1008 having a seconddiameter d2, where d2 is smaller than d1. (See FIG. 10D and The interiorsurface of the holes 1006 and 1010 may then be plated with a conductivematerial. (FIGS. 10E and 10F)

Referring to FIG. 10F, after the first and second holes 1010 and 1006are plated 1011, a third drill 1014, having a third diameter d3, maypass (drill) through the first and second holes 1006 and 1010. The thirddiameter is larger than the second diameter d2. As the third drill 1014has a larger diameter then the second hole 1006 but a smaller diameterthan the hole 1010, the conductive material in the first hole 1010 maybe left intact but the majority of the conductive material on the firsthole 1006 may be removed, i.e. the stub area is removed. As shown inFIG. 10G, the third drill 1014 is not drilled all the way through theplated first hole 1006 leaving a small portion or small stub 1016 of theplating resist on the inner surface of the first hole 1006. A small stub1016 may remain because when drilling upwardly from the lower surface1001 b of the PCB, the drill bit has no pressure and creates fluctuatingas well as copper peel off. Also, short depth drill has higher depthaccuracy than long depth drill, so controlling stub length by shorterdepth drill may provide shorter stub length than longer depth drill. Forinstance, this process may provide a shorter stub than when drilledusing the third drill from the second (or top) hole 1010 to the first(or bottom) hole 1006 as in FIG. 2 and FIG. 3 where the stub length iscontrolled by a long depth second drill. As a result, the through hole1000, as shown in FIG. 10G, may have an upper conductive portion 1002, alower non-conductive portion 1004 and a small stub 1016. (See FIG. 10G)

FIGS. 11A1, 11A2 and 11B-11G are illustrations of the differentfabrication stages for forming a plated through hole (or via) 1100 andremoval of a short stub in a printed circuit board (PCB) 1101, accordingto one aspect. The printed circuit board has an upper surface 1101 a andan opposing lower surface 1101 b.

The location where a signal goes through the PTH, or where a first layer1102 is connected to a second layer 1104 in the PCB 1100, may berepresented as a target layer (T). According to one aspect, the firstlayer 1102 may include an upper hole 1108 drilled using a long depthdrill while the second layer 1104 may include a lower hole 1106 drilledusing a short depth drill. The long depth drill has a length that isless than or equal to the distance from the bottom of the first layer1102 to the target layer T prior to plating.

As shown in FIG. 11A-1, the lower hole 1106 may be formed using a shortdepth drill having a diameter of d1, such that the top of the lower hole1106 may be just above the target layer T. In other words, the shortdepth drill passes through the target layer T when drilling from thefirst layer 1104 upward. Next, the upper hole 1108 may be drilled, fromthe upper surface 1101 a to the top of the lower hole 1106 using a longdepth drill having a diameter of d2, where d2 is larger than d1. (SeeFIG. 11B-1) The interior surface of the upper and lower holes 1108 and1106 may be then plated 1112 with a conductive material. (FIG. 11C)

After the upper and lower holes 1110 and 1106 are plated 1112, a thirdhole 1114, having a diameter d3, which is greater than d1 but less thand2, is back drilled to slightly below the target layer T. As d3 isgreater than d1, the conductive material 1112 in the upper hole 1108 maybe left intact but the majority of the conductive material on the lowerhole 1106 may be removed, i.e. the stub area is removed. As shown inFIG. 11D, the third hole 1114 is not drilled, all the way through theplated lower hole 1106 leaving a small portion or small stub 1116 of theplating resist on the inner surface of the first hole 1106. A small stub1116 may remain because when drilling upwardly from the lower surface1101 b of the PCB, the drill bit has no pressure and creates fluctuatingas well as copper peel off. Also, a short depth drill has higher depthaccuracy than long depth drill, so controlling stub length by shorterdepth drill may provide shorter stub length than longer depth drill. Forinstance, this process may provide a shorter stub than when drilled withthe third drill from upper hole 1110 to lower hole 1106 as in FIG. 2 andFIG. 3 where the stub length is controlled by a long depth second drill.As a result, the through hole 1100, as shown in FIG. 11D may have anupper conductive portion, a lower non-conductive portion and a smallstub 1116.

Alternatively, as shown in FIG. 11A-2, the upper hole 1108 may bedrilled first, using the long depth drill, down to the Target Layer T.Next, the small depth drill may be used to drill the upper hole 1108such that it connects with the lower hole 1106 at or below the targetlayer T. Next, the lower hole 1106 may be drilled, from the lowersurface 1101 b to the bottom of the lower hole 1106 using a short depthdrill having a diameter of d2, where d2 is smaller than d1. (See FIG.12B-2) The interior surface of the upper and lower holes 1108 and 1106may be then plated 1112 with a conductive material. (FIG. 11C)

After the upper and lower holes 1110 and 1106 are plated 1112, a thirdhole 1114, having a diameter d3, which is greater than d1 but less thand2, is back drilled to slightly below the target layer T. As d3 isgreater than d1, the conductive material 1112 in the upper hole 1108 maybe left intact but the majority of the conductive material on the lowerhole 1106 may be removed, i.e. the stub area is removed. As shown inFIG. 11D, the third hole 1114 is not drilled, all the way through theplated lower hole 1106 leaving a small portion or small stub 1116 of theplating resist on the inner surface of the first hole 1106. A small stub1116 may remain because when drilling upwardly from the lower surface1101 b of the PCB, the drill bit has no pressure and creates fluctuatingas well as copper peel off. Also, a short depth drill has higher depthaccuracy than long depth drill, so controlling stub length by shorterdepth drill may provide shorter stub length than longer depth drill. Forinstance, this process may provide a shorter stub than when drilled withthe third drill from upper hole 1110 to lower hole 1106 as in FIG. 2 andFIG. 3 where the stub length is controlled by a long depth second drill.As a result, the through hole 1100, as shown in FIG. 11D may have anupper conductive portion, a lower non-conductive portion and a smallstub 1116.

FIG. 12 illustrates a method for forming a plated through hole in alaminated PCB, according to one aspect. In the method, a first hole,having a first diameter, may be drilled from a lower surface of the PCBto a predetermined depth using a first drill having a first diameter1202. For example, the predetermined depth may be half way through thePCB. Next, a second hole may be drilled from an upper surface of the PCBto a top of the first hole using a second drill having a seconddiameter, where the second diameter is larger than the first diameter1204. The first hole and the second hole may then be plated with aconductive material 1206. Next, a third hole, having a third diameter,may be drilled to the first hole from the lower surface of the PCB usinga third drill, where the third diameter is larger than the firstdiameter 1208. As the third diameter is larger than the first diameterand smaller than the second diameter, when the third hole is drilled theplated material may be removed from the inner surface of the first hole.That is, the stub is removed. As a result, a through hole having anupper conductive portion and a lower non-conductive portion may beformed.

In another aspect, as illustrated in FIG. 13, a first hole, having afirst diameter, may be drilled from an upper surface of the PCB to apredetermined depth using a first drill having a first diameter 1302.For example, the predetermined depth may be half way through the PCB.Next, a second hole may be drilled through the first hole of the PCB toa bottom of the first hole using a second drill having a seconddiameter, where the second diameter is smaller than the first diameter1304. The first hole and the second hole may then be plated with aconductive material 1306. Next, a third hole, having a third diameter,may be drilled to the first hole from the lower surface of the PCB usinga third drill, where the third diameter is larger than the seconddiameter 1308. As the third diameter is larger than the second diameter,when the third hole is drilled the plated material is removed from theinner surface of the second hole. That is, the stub is removed. As aresult, a through hole having an upper conductive portion and a lowernon-conductive portion may be formed.

In another aspect, as illustrated in FIG. 14, a first hole may bedrilled from a lower surface of the PCB to a predetermined depth using afirst drill having a first diameter 1402. For example, the predetermineddepth may be half way through the PCB. Next, a second hole may bedrilled from an upper surface of the PCB to a top or upper end of thefirst hole using a second drill having a second diameter, where thesecond diameter is larger than the first diameter 1404. The first holeand the second hole may then be plated with a thin conductive materialsuch as electroless plating catalyst or electroless copper 1406. Next, athird hole, having a third diameter, may be drilled to the first holefrom the lower surface of the PCB using a third drill, where the thirddiameter may be larger than the first diameter 1408. A conductivematerial, such as copper, may then be added to the remaining conductivearea by electrolytic plating 1410. As the third diameter is larger thanthe first diameter, when the third hole is drilled the plated materialmay be removed from the inner surface of the first hole. That is, thestub is removed. As a result, a through hole having an upper conductiveportion and a lower non-conductive portion may be formed.

In another aspect, as illustrated in FIG. 15, a first hole is drilledfrom an upper surface of the PCB to a predetermined depth using a firstdrill having a first diameter 1502. For example, the predetermined depthis half way through the PCB. Next, a second hole is drilled through thefirst hole to a bottom of the first hole using a second drill having asecond diameter, where the second diameter is smaller than the firstdiameter 1504. The first hole and the second holes are then plated witha thing conductive material such as electroless plating catalyst orelectroless copper 1506. Next, a third hole, having a third diameter, isdrilled to the first hole from the lower surface of the PCB using athird drill, where the third diameter is larger than the second diameter1508. Conductive material, such as copper, is then added to theremaining conductive area by electrolytic plating 1510. As the thirddiameter is larger than the first diameter and smaller than the seconddiameter, when the third hole is drilled the plated material is removedfrom the inner surface of the second hole. That is, the stub is removed.As a result, a through hole having an upper conductive portion and alower non-conductive portion is formed.

Forming a Via without a Stub

In another aspect, the present disclosure provides a method of usingdifferent diameter drills within a PCB and a conductive material filledcore having a connecting inner layer trace (target layer) for forming avia without a stub. FIGS. 16A-16H illustrate the different fabricationstages for forming a plated through hole (or via) using differentdiameter drills within a PCB and a conductive material filled corehaving a connecting inner layer trace (target layer) for forming a viawithout a stub, according to one aspect.

In the method, four different drill sizes may be utilized to drillthrough an upper surface 1600 a and lower surface 1600 b of a PCB 1600passing through embedded conductive material in the PCB 1600. First, afirst hole 1602 is drilled in a core 1604 having a diameter of d1 (SeeFIGS. 16A-16D) and is filled with conductive material 1606, such asconductive paste or plated copper. Next, a second hole 1608, where thesecond hole 1608 is larger than the first hole 1602, is drilled withdiameter of d2 after lamination until reaching the embedded conductivematerial 1606. Next, a third hole 1610, having a diameter of d3, isdrilled by drilling through the second hole 1608, where diameter d3 issmaller than d1 and d2. Next, the second and third holes 1608 and 1610are plated with a conductive material 1612 such as copper. Next, theplated conductive material is removed from the surface of the third hole1610 by drilling through the second hole 1608 forming a fourth 1614having a diameter of d4, where d4 is larger than d3 and smaller than d1and d2. (See FIGS. 16E-16H)

In another aspect, the present disclosure provides a method of usingdifferent diameter drills within a PCB and a conductive material filledcore having a connecting inner layer trace (target layer) and adjacentconductive material filled prepreg for forming a via without a stub.FIGS. 17A-17H illustrate the different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB and a conductive material filled core having a connecting innerlayer trace (target layer) and adjacent conductive material filledprepreg for forming a via without a stub, according to one aspect.

In the method, four different drill sizes may be utilized to drillthrough an upper surface 1700 a and lower surface 1700 b of a PCB 1700passing through embedded conductive material in the PCB 1700. First, afirst hole 1702 is drilled in a core 1704 and prepreg 1706 having adiameter of d1 and filled with conductive material 1708, such asconductive paste or plated copper. Next, a second hole 1710, where thesecond hole 1710 is larger than the first hole 1702, is drilled having adiameter of d2 after lamination, until reaching the embedded conductivematerial 1708. Next, a third hole 1712, where the third hole is smallerthan the second hole 1710, having a diameter d3 is drilled by drillingthrough the second hole 1710, where d3 is smaller than d1 and d2. Next,the second hole 1710 and the third hole 1712 are plated over by aconductive material 1714 such as copper. Next, the plated conductivematerial is removed from the surface of the third hole 1712 by drillingthrough the second hole 1710 forming a fourth hole 1716 having adiameter of d4, where d4 is larger than d3 and smaller than d1 and d2.(See FIGS. 17E-17H)

In another aspect, the present disclosure provides a method of usingdifferent diameter drills within a PCB and multiple conductive materialfilled cores having a connecting inner layer trace (target layer) andadjacent conductive material filled prepreg for forming a via without astub. FIGS. 18A-18H illustrate different fabrication stages for forminga plated through hole (or via) using different diameter drills within aPCB having a conductive material filled prepreg between conductivematerial filled cores; and having a connecting inner layer trace (targetlayer) for forming a via without a stub, according to one aspect.

In the method, three different drill sizes may be utilized to drillthrough an upper surface 1800 a and lower surface 1800 b of a PCB 1800passing through embedded conductive material in the PCB 1800. First, afirst hole 1802 is drilled in the core 1804 and prepreg 1806 having adiameter of d1 and filled with conductive material 1808, such as aconductive paste or plated copper 1808. Next, a second hole 1810, havinga diameter of d2, is drilled after lamination, until reaching theembedded conductive material 1808. Next, a third hole 1812, having adiameter of d3, is drilled through the second hole 1810, where d3 issmaller than d2 and d1. The second and third holes 1810 and 1812 arethen plated over with conductive material 1814, such as copper. Next,plated conductive material is removed from the surface of the third hole1812 by drilling down through the second hole 1810 forming a fourth hole1816 having a diameter of d4, where d4 is larger than d3 and smallerthan d1 and d2. (See FIGS. 18E-18H)

FIGS. 19A-19G the different fabrication stages for forming a platedthrough hole (or via) using different diameter drills within a PCBhaving a conductive material filled prepreg and solid core as well ashaving a connecting inner layer trace (target layer) for forming a viahole without a stub.

In the method, four different drill sizes are utilized to drill throughan upper surface and lower surface of a PCB 1900 passing throughembedded conductive material in the PCB 1900. A first hole 1902, havinga diameter d1, is drilled in the pregreg 1906 and filled with conductivematerial 1908 such as conductive paste.

Next, a second hole 1910, having a diameter of d2, is drilled afterlamination until reaching the top of the embedded conductive material1908. Next, a third hole 1912 having a diameter d3 is drilled, where d3is smaller than d1 and d2. Next, the second hole 1910 and the third hole1912 are plated over by a conductive material 1914 such as copper. Thethird hole 1912 is then is back drilled to remove the conductivematerial from the inner surface of the third hole 1912 and form a fourthhole having a diameter of d4, where d4 is larger than d3 and smallerthan d1. (See FIGS. 19E-19G)

FIGS. 20A-20H illustrate the different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB having a conductive material filled prepreg 2004 between aconductive material filled core 2006 and a solid core 2008; and aconnecting inner layer trace (target layer) for forming a via without astub.

In the method, five different drill sizes are utilized to drill throughan upper surface and lower surface of a PCB 2000 passing throughembedded conductive material in the PCB 2000. A first hole 2002 and asecond hole 2003 are drilled in the pregreg 2004 and the core 2006having a diameter of d1 and d2, respectively (FIG. 20A), filled withconductive material 2010, such as conductive paste or plated copper, andan inner layer circuit is formed on the core 2006 (FIG. 20B). Then thecores and prepreg are laminated aligning stacked conductive paste in thecore and prepreg to form PCB 2000. (FIG. 20D)

Next, a third hole 2012, having a diameter of d3, is drilled afterlamination until reaching the top of the embedded conductive material2010. Next, a fourth hole 2014 having a diameter d4 is drilled, where d4is smaller than d3. Next, the third hole 2012 and the fourth hole 2014are plated over by a conductive material 2016 such as copper. Next, thefourth hole 2014 is back drilled to remove the conductive material andform a fifth hole having a diameter of d5, where d5 is larger than d4and smaller than d3. (See FIGS. 20E-20H) Alternatively, the fifth drillmay be applied after conductive seeding prior to electrolytic plating.

In another aspect, the present disclosure provides a method of usingdifferent diameter drills within a PCB and a conductive material filledcore having a connecting inner layer trace (target layer) and adjacentconductive material filled prepregs for forming a via without a stub.FIGS. 21A-21H illustrate the different fabrication stages for forming aplated through hole (or via) using different diameter drills within aPCB and multiple conductive material filled cores having a connectinginner layer trace (target layer) and adjacent conductive material filledprepregs for forming a via without a stub.

In the method, six different drill sizes are utilized to drill throughan upper surface and lower surface of a PCB 2100 passing throughembedded conductive material in the PCB 2100. A hole 2102 b, having adiameter d2, is drilled in the core 2104 b and holes 2102 a and 2102 c,having a diameter d1 and d3 respectively, are drilled in the first andsecond prepregs 2104 a and 2104 c, and filled with conductive material2106, such as a conductive paste or plated copper. The core 2104 b isformed on a circuit on the surface 2108. The materials are laminated tomake a PCB 2100 aligning conductive material filled holes. Next, afourth hole 2110, having a diameter d4, is drilled after lamination,until reaching the embedded conductive material 2106. Next, a fifth hole2112, having a diameter d5, is drilled through the PCB 2100 and theembedded conductive material 2108, where the fourth hole 2110 and thefifth hole 2112 are plated over with conductive material 2114, such ascopper. Next, the fifth hole 2112 is back drilled to remove theconductive material from the fifth hole 2112, while maintaining theconductive material in the fourth hole 2110, and forming a sixth hole2116 having a diameter of d6, where d6 is larger than d5 and smallerthan d1, d2, d3 and d4. (See FIGS. 21E-21H) Alternatively, back drillingmay proceed after conductive seeding for electrolytic plating, such aselectroless plating catalyst or electroless copper, prior toelectrolytic plating.

Formation of Plated Through Hole with High Aspect Ratio

Referring to FIGS. 22A-22G, the different fabrication stages for forminga plated through hole (or via) 2220 having a high aspect ratio in aprinted circuit board (PCB) 2201 are illustrated.

As described below, a three (3) drill step process is used to form theplated through hole. In the first step, a first hole 2206 is drilled,from the lower surface 2201 b of the PCB, using a first drill 2208having a first diameter d1. The first hole 2206 is drilled approximatelyhalf way through the PCB 2201. (See FIGS. 22A and 22B) Next, the seconddrill 2209, having a second diameter d2, is used to drill, from theupper surface 2201 a of the PCB, down to a top of the first hole 2206forming a second hole 2210. An offset 2212 may be created at the end ofthe second hole 2210 and the beginning of the first hole 2206. The firsthole 2206 and the second hole 2210 together form a hole extendingthrough the entire vertical length of the PCB. (See FIG. 22D)

Next, a third drill 2214 is used to drill from the lower surface 2201 bupwards (or alternatively the upper surface 2201 a downwards) throughthe first hole 2206 and the second hole 2210. The third drill 2214 has athird diameter d3 which is larger than the first diameter d1 of thefirst drill 2208 and the second diameter d2 of the second drill 2208. Asa result, a smooth hole 2216 is formed that extends through the entirevertical length of the PCB 2201. (See FIGS. 22E-22F) The smooth hole2216 is then plated 2218 with a conductive material forming a platedthrough hole 2220. (See FIG. 22G)

FIG. 23 illustrates a method for forming a plated through hole, having ahigh aspect ratio, in a PCB, according to one aspect. In the method, afirst hole is drilled, from a first surface of the PCB to apredetermined depth using a first drill having a first diameter 2302.For example, the predetermined depth is half way through the PCB. Next,a second hole is drilled, using a second drill having a second diameter,from a second surface of the PCB, the second surface opposite the firstsurface, to an upper end of the first hole. These holes may create anoffset between a lower end of the second hole and the upper end of thefirst hole due to drill process accuracy 2304. Next, a third hole isdrilled, using a drill having a third diameter which is larger than boththe first and the second drill diameter, through the first and secondholes smoothing the offset between the first and second holes 2306. Thethird hole is then plated with a conductive material forming a platedthrough hole 2308.

In another aspect, the present disclosure provides a method of forming aplated through hole, having a high aspect ratio, in a PCB. (See FIGS.24A-24D) In the method, two different drill sizes may be utilized todrill through an upper surface 2400 a and lower surface 2400 b of a PCB.First, a first hole 2402 is drilled approximately half way through thePCB 2400 using a first drill 2404. Next, the first drill 2404 is used todrill a second hole 2406 from the bottom surface 2400 b of the PCB 2400to the bottom of the first hole 2402. An offset 2408 is created betweenthe first hole 2402 and the second hole 2406 in the PCB 2400. A seconddrill 2408, where the second drill 2408 is larger than the first drill2404 (i.e. has a larger diameter), is then used to drill a third hole2410 through both the first hole 2402 and the second hole 2406. As thesecond drill 2408 has a diameter larger than the diameter of the smalldrill 2404, the first and second holes 2402 and 2406 work as a guide andare smoothed by the large diameter drilling. A conductive material 2412is then plated on the inner surface of the hole 2410 forming a platedthrough hole which extends the vertical length of the PCB.

While certain exemplary aspects have been described and shown in theaccompanying drawings, it is to be understood that such aspects aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

1. A method for forming a plated through hole in a printed circuitboard, comprising: drilling a first hole having a first diameter from alower surface of the printed circuit board; drilling a second holehaving a second diameter through an upper surface of the printed circuitboard to the first hole of the printed circuit board, the upper surfaceopposite the lower surface; plating the first hole and the second holewith a conductive material; and drilling a third hole having a thirddiameter through the first hole and the second hole.
 2. The method ofclaim 1, wherein the printed circuit board has a plurality of platedthrough holes formed therein.
 3. The method of claim 1, wherein thesecond hole is drilled a predetermined depth; and wherein thepredetermined depth is shorter than half the vertical distance of thePCB.
 4. The method of claim 3, wherein the second diameter is largerthan the first diameter; and wherein the third diameter is larger thanthe second diameter and smaller than the first diameter.
 5. The methodof claim 3, wherein the third hole is drilled from the upper surface tothe lower surface of the printed circuit board.
 6. The method of claim3, wherein the second diameter is larger than the first diameter; andwherein the third diameter is smaller than the second diameter andlarger than the first diameter.
 7. The method of claim 3, wherein thesecond diameter is larger than the first diameter; wherein a top of thethird hole to a bottom of the second hole remains undrilled; and whereinand the third diameter is larger than the first diameter.
 8. The methodof claim 7, further comprising adding copper to any remaining conductivesurface by electrolytic plating.
 9. The method of claim 1, wherein thefirst hole is drilled a predetermined depth; and wherein thepredetermined depth is shorter than half the vertical distance of theprinted circuit board.
 10. The method of claim 9, further comprising:plating the third hole with a conductive material forming a platedthrough hole; and wherein the second hole is drilled to a top of thefirst hole; and wherein the third diameter is larger than the firstdiameter and the second diameter.
 11. A method for forming a platedthrough hole in a printed circuit board, comprising: drilling a firsthole having a first diameter from a lower surface of the printed circuitboard; drilling a second hole having a second diameter through an uppersurface of the printed circuit board to the first hole of the printedcircuit board, the second hole drilled a predetermined depth which isshorter than half the vertical distance of the PCB, and the uppersurface opposite the lower surface; plating the first hole and thesecond hole with a conductive material; drilling a third hole having athird diameter through the first hole and the second hole; and addingcopper to any remaining conductive surface by electrolytic plating. 12.The method of claim 1, wherein the printed circuit board has a pluralityof plated through holes formed therein.
 13. The method of claim 11,wherein the second diameter is larger than the first diameter; andwherein the third diameter is larger than the second diameter andsmaller than the first diameter.
 14. The method of claim 11, wherein thesecond diameter is larger than the first diameter; and wherein the thirddiameter is larger than the second diameter and smaller than the firstdiameter.
 15. The method of claim 11, wherein the third hole is drilledfrom the upper surface to the lower surface of the printed circuitboard.
 16. The method of claim 11, wherein the second diameter is largerthan the first diameter; and wherein the third diameter is smaller thanthe second diameter and larger than the first diameter.
 17. The methodof claim 11, wherein the second diameter is larger than the firstdiameter; wherein a top of the third hole to a bottom of the second holeremains undrilled; and wherein and the third diameter is larger than thefirst diameter.
 18. A method for forming a plated through hole in aprinted circuit board, comprising: drilling a first hole having a firstdiameter from a lower surface of the printed circuit board; drilling asecond hole having a second diameter through an upper surface of theprinted circuit board to the first hole of the printed circuit board,the upper surface opposite the lower surface; plating the first hole andthe second hole with a conductive material; drilling a third hole havinga third diameter through the first hole and the second hole; and platingthe third hole with a conductive material forming a plated through hole;wherein the second hole is drilled to a top of the first hole; andwherein the third diameter is larger than the first diameter and thesecond diameter.
 19. The method of claim 18, wherein the second diameteris larger than the first diameter; and wherein the third diameter islarger than the second diameter and smaller than the first diameter. 20.The method of claim 18, wherein the second diameter is larger than thefirst diameter; and wherein the third diameter is larger than the seconddiameter and smaller than the first diameter.